It's really pretty easy to use the hydrometer; just follow these simple steps:
1. Sanitize the hydrometer, wine thief, and test jar.
2. Place test cylinder on flat surface.
3. Draw a sample of "clean" beer with the wine thief - avoid testing samples that contain solid particles, since this will affect the readings.
4. Fill the test jar with enough liquid to just float the hydrometer - about 80% full.
5. Gently lower the hydrometer into the test jar; spin the hydrometer as you release it, so no bubbles stick to the bottom of the hydrometer (this can also affect readings).
6. Making sure the hydrometer isn't touching the sides of the test jar and is floating freely, take a reading across the bottom of the meniscus (see diagram to the left). Meniscus is a fancy word for the curved surface of the liquid.
Most hydrometers used in testing are calibrated for with a reference temperature of 60ºF. These hydrometers will be inscribed with "60ºF/60ºF" on their necks denoting that the reference density was measured at 60ºF and the sample should be at 60ºF.
You can adjust the reading for any liquid temp. with this calculator.
http://www.fermsoft.com/hydroCorrect.html
You imput the gravity reading, the temp of the liquid and the calibration of your hydrometer (either 60 or 68) and it will correct for you.